This invention relates to a mirror of the class constituted fundamentally of a hard and transparent synthetic resin plate and a thin reflective film of a metal coated on one side of the resin plate. For example, a mirror according to the invention is suitable for use as a rearview mirror in automobiles or other vehicles.
Mirror made of synthetic resins are gaining increasing acceptance for outdoor applications. Since resin mirros are shatter-proof, they are considered preferred over conventional glass mirros for use within cars. However, mirrors within automobiles must satisfy certain safety requirements for optical parameters such as reflectance, warpage and distortion.
U.S. Pat. No. 4,385,804 issued to Tamura et al. on May 31, 1983 discloses a mirror comprising:
a transparent plate of a synthetic resin;
a surface-hardening film coated at least on a front surface of the resin plate;
a reflective film of a metal resistant to moisture deposited on the back side of the resin plate; and
a back-coat layer closely covering the outer surface of the reflective film,
the surface-hardening film, the reflective film and the back-coat layer being all substantially equally permeable to humidity.
As shown above, the Tamura et al. patent uses a surface-hardening film, a reflective film and a back-coat layer having substantially the same permiability to moisture. Therefore, the amount of moisture that enters the front side of the resin plate coated with only the surface-hardening film is not equal to that of moisture entering the back side of the resin coated with the above mentioned three layers, causing warpage or distortion in the resin plate.
The present inventor made various studies to eliminate these defects of the Tamura patent and found unexpectedly that either tin or a binary alloy thereof with less than 40%, of tin, of copper that was considered poorly resistant to corrosion has good properties suitable for use as a material for the reflective layer. The present invention has been accomplished on the basis of this finding.